FireEye recently detected malicious Microsoft Office RTF documents that leverage a previously undisclosed vulnerability. This vulnerability allows a malicious actor to execute a Visual Basic script when the user opens a document containing an embedded exploit. FireEye has observed several Office documents exploiting the vulnerability that download and execute malware payloads from different well-known malware families.

FireEye shared the details of the vulnerability with Microsoft and has been coordinating for several weeks public disclosure timed with the release of a patch by Microsoft to address the vulnerability. After recent public disclosure by another company, this blog serves to acknowledge FireEye’s awareness and coverage of these attacks.

The attack involves a threat actor emailing a Microsoft Word document to a targeted user with an embedded OLE2link object. When the user opens the document, winword.exe issues a HTTP request to a remote server to retrieve a malicious .hta file, which appears as a fake RTF file. The Microsoft HTA application loads and executes the malicious script. In both observed documents the malicious script terminated the winword.exe process, downloaded additional payload(s), and loaded a decoy document for the user to see. The original winword.exe process is terminated in order to hide a user prompt generated by the OLE2link.

The vulnerability is bypassing most mitigations; however, as noted above, FireEye email and network products detect the malicious documents. Microsoft Office users are recommended to apply the patch as soon as it is available.